The present invention relates to a process for the thermal treatment of the precursor material of a catalytically active material in a rotary tube furnace through which a gas stream flows.
In this document, the term catalytically active material is to be understood as meaning solids which are used in chemical reactions, for example in chemical gas-phase reactions, in addition to the reactants, either to reduce the temperature required for carrying out the chemical reaction and/or to increase the selectivity of the formation of the desired product. The chemical reaction takes place as a rule at the surface (interface) of the catalytically active material.
Examples of such heterogeneously catalyzed reactions, which can be carried out in principle both in the liquid phase and in the gas phase, are heterogeneously catalyzed hydrogenations, dehydrogenations and oxydehydrogenations, but also partial oxidations and partial ammoxidations. There, the active material can be used both in powder form and in the form obtained after shaping to give geometric moldings (the latter can be effected, for example, by introduction (absorption) into the inner surface of a premolded support (the term supported catalyst is then used), by application to the outer surface of a premolded support (the term coated catalyst is then used) or by compression (the term unsupported catalysts is then used)). The shaping can be applied to the precursor material itself or only to the catalytically active material.
A complete oxidation of an organic compound (for example a saturated or unsaturated hydrocarbon, an alcohol or an aldehyde) is understood in this document as meaning that all the carbon contained in the organic compound is converted into oxides of carbon (CO, CO2).
All other reactions of organic compounds with oxygen (including the oxydehydrogenations) are regarded in this document as partial oxidations. The partial ammoxidation differs from the partial oxidation through the additional presence of ammonia.
It is now generally known that catalytically active materials, molded or unmolded, are generally obtainable by producing a precursor material which as a rule is catalytically inactive or may have reduced activity and may be molded or still unmolded, and exposing said precursor material to a specific gas atmosphere at elevated temperatures.
For example, DE-A 10211275 describes, in its working examples, the activation of a precursor material of a dehydrogenation catalyst at elevated temperature (500° C.) in changing gas streams (hydrogen, air, nitrogen) and its use in catalysts for the heterogeneously catalyzed dehydrogenation of hydrocarbons in the gas phase.
Similarly, EP-A 529853, EP-A 318295, EP-A 608838, WO 01/96270, EP-A 731077, EP-A 1260495, EP-A 1254709, EP-A 1192987, EP-A 962253, EP-A 1262235, EP-A 1090684, DE-A 19835247, EP-A 895809, DE-A 10261186, EP-A 774297, WO 02/24620, EP-A 668104, DE-A 2161450, EP-A 724481, EP-A 714700, DE-A 10046928 and DE-A 19815281 describe the thermal treatment of precursor materials of multielement oxide active materials in a very wide range of gas atmospheres. In these publications, the resulting multielement oxide active materials are recommended and used as catalytic active materials in catalysts for a very wide range of heterogeneously catalyzed partial gas-phase oxidations and gas-phase ammoxidations of organic compounds.
In principle, such thermal treatments can be carried out in a very wide range of furnace types, for example tray furnaces, rotary furnaces, belt calciners, fluidized-bed ovens or shaft furnaces.
Of increasing importance (cf. WO 02/24620) is that all of the precursor material to be thermally treated is treated under very uniform conditions in order to obtain a total amount of active material which has very uniform characteristics.
For example, amounts of active material which have very uniform characteristics are more suitable for heterogeneous gas-phase catalyses with high reactant loading of the active material since they permit particularly uniform thermal reaction conditions over a reactor cross section.
Against this background, WO 02/24620 recommends carrying out the thermal treatment of such precursor materials by means of a special belt calcination apparatus. A disadvantage of a belt calcination, however, is that it is effected on stationary beds of the precursor material. However, temperature gradients which prevent a uniform thermal treatment over the entire precursor material to be thermally treated usually form within such a stationary bed.
A thermal treatment of the precursor material in a moving bed, as is present in thermal treatments in rotary tube furnaces, would be preferable in comparison. Owing to the continuous mobility of the precursor material in rotary tube furnaces, said material forms a continuously self-homogenizing bed, within which, for example, hot spots or cold spots do not form (most thermal treatments of precursor materials involve exothermic or endothermic processes which lead to the formation of hot spots (locations of maximum temperature within the thermally treated precursor material) or to the formation of cold spots (locations of minimum temperature within the thermally treated precursor material)). However, both excessively high and excessively low temperatures adversely affect the catalytic properties of the active material.
Another point of view is that the predominant number of thermal treatments of precursor materials is accompanied by thermal decomposition processes of chemical components contained in the precursor material, with formation of gaseous decomposition products which may have an advantageous or disadvantageous effect on the resulting quality of the active material. In both cases, self-homogenization in a moving bed would be advantageous.
A thermal treatment of precursor material of an active material in a rotary tube furnace is suggested, inter alia, in DE-A 19815281 (e.g. example 1), DE-A 10046928 (e.g. preparation example 1) and EP-A 714700 (working examples).
Surprisingly, such a thermal treatment in a rotary tube furnace is carried out industrially in such a way that the angle of inclination of the rotary tube with the horizontal is adjusted to a value other than zero. The highest point of the rotary tube is the point of introduction of the precursor material and the lowest point is the location of the material discharge. The rotary tube is operated continuously, i.e. the precursor material to be thermally treated is fed continuously to one side of the rotary tube, transported continuously in the rotary tube from the highest to the lowest point and continuously discharged there. Along the way through the rotary tube, the precursor material usually undergoes thermal treatment.
A disadvantage is that such a continuous procedure permits only comparatively short residence times of the material to be thermally treated in the rotary tube.
For establishing the desired gas atmosphere, usually a corresponding gas stream is passed through the rotary tube, countercurrently to the transported precursor material. In the simplest case, said gas stream may consist of air but in other cases, inter alia, also of useful gases (for example reducing gases, such as hydrogen or ammonia, or inert gases, such as nitrogen).
A disadvantage of the procedure described is the comparatively high requirement of such gases, which are not used further after leaving the rotary tube. Another disadvantage is that gases which are formed in the material by thermal decomposition and have a favorable effect are discharged with the gas stream and can no longer display their advantageous effect (e.g. NH3 formed from NH4+, NO2 formed from NO3−, or CO2 or CO formed from CO32−,). Such an advantageous effect may be, for example, a reducing effect.
The temperature desired in the material present in the rotary tube is usually generated indirectly by bringing the rotary tube wall to a certain temperature from the outside.
In order to avoid pronounced radial and axial temperature gradients in the rotary tube, it would be desirable to feed the gas stream passed through the rotary tube into the rotary tube after said gas stream has been preheated to the temperature desired for the material in the rotary tube.
Usually, this heat content of the gas stream would not be used further on leaving the rotary tube, which is a disadvantage.